Effects of Trophic Status on Mercury Methylation Pathways in Peatlands

Abstract:

Methyl mercury (MeHg) is a bioaccumulative toxicant. It was believed to be produced by sulfate (SO₄)- and iron- reducing bacteria (SRB and FeRB), but recent studies suggest that organisms that possess the gene cluster (hgcAB) can methylate Hg, which includes other microbial groups besides SRB and FeRB. Many areas known to accumulate high levels of MeHg are freshwater wetlands that lack sufficient electron acceptors to support the production of MeHg. To test the hypothesis that oligotrophic wetlands are able to methylate Hg by pathways that are not respiratory, peat was collected from three wetland sites in Alaska and three in Massachusetts that represented a trophic gradient. We determined rates of gas (CH₄, CO₂, H₂) and LMW organic acid (formate, acetate, propionate, butyrate) formation, and rates of Hg methylation using the short-lived radioisotope ¹⁹⁷Hg (half life 2.67 days). Two temperate sites exhibited strong terminal respiration (methanogenesis) and syntrophy, while the Alaskan sites and an oligotrophic temperate site exhibited low rates of both. Primary fermentation was an important process in the latter sites. Hg methylation was most active at the minerotrophic sites that exhibited active syntrophy and methanogenesis. Methylation decreased greatly in the presence of a methanogenic inhibitor and was stimulated by H₂ indicating that methanogens and perhaps syntrophs were primary methylators. In the oligotrophic sites, the addition of SO₄ stimulated methylation while a SO₄ reduction inhibitor decreased methylation. There was no evidence of SO₄ reduction in these samples suggesting that methylation was conducted by SRB that were metabolizing via fermentation and not SO₄ reduction. While further studies are required to decipher the role of syntrophs including SRB varieties such as Syntrophobacter sp., these results indicate that fermentative bacteria may be able to significantly methylate Hg in wetlands that do not support anaerobic respiration.